The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. In October 2009 ATSC published an ATSC Mobile DTV Standard, Parts 1-8 as Document A/153, referred to hereinafter simply as “A/153” and incorporated herein by reference. A/153 is directed to transmitting ancillary signals in time division multiplex with 8VSB DTV signals, which ancillary signals are designed for reception by mobile receivers and by hand-held receivers. The ancillary data are randomized and subjected to transverse Reed-Solomon (TRS) coding before concatenated convolutional coding (CCC) that uses the 12-phase 2/3 trellis coding of 8VSB as inner convolutional coding. A/153 prescribes serial concatenated convolutional coding (SCCC), but parallel concatenated convolutional coding (PCCC) that uses the 12-phase 2/3 trellis coding of 8VSB as inner convolutional coding can be used instead. U.S. patent application Ser. No. 12/580,534 filed for A. L. R. Limberg on 16 Oct. 2009 and titled “Digital television systems employing concatenated convolutional coded data” describes such PCCC that uses single-phase outer convolutional coding. This application was published 22 Apr. 2010 as US-2010-0100793-A1. Provisional U.S. Pat. App. Ser. No. 61/335,246 filed for A. L. R. Limberg on 4 Jan. 2010 and titled “Coding and decoding of RS Frames in 8VSB digital television signals intended for reception by mobile/handheld receivers” describes PCCC that uses 12-phase outer convolutional coding instead.
A/153 contemplated the use of receivers designed for selective reception of M/H Groups with the receiver being in large measure turned off when other M/H Groups would otherwise be received. When considering the design of such a receiver, the inventor discovered that the technical experts preparing A/153 apparently failed to notice a shortcoming with regard to providing for such selective reception. A/153 contains no provision for placing the parity bits of inner convolutional coding into prescribed initial states at the beginning of each M/H Group. Accordingly, the receiver designed for selective reception of M/H Groups has to rely on chance for correct synchronization of the decoder for the inner convolutional coding to occur at the outsets of the Groups selected for reception. This problem arises because the 2/3 trellis coding used as the inner outer convolutional coding continues through the ordinary 8VSB signals between M/H Groups.
This problem can be overcome by modifying certain bytes at the beginning of CCC in each M/H Group to reset the twelve 2/3 trellis encoders at the 8VSB DTV transmitter to prescribed states as CCC continues within the Group. The inventor observed that there were dummy bytes in the beginning of each M/H Group as CCC started. The inventor ascertained that these dummy bytes are so positioned within the M/H Group that there is the possibility that they can be modified to reset the 12-phase 2/3 trellis encoder at the 8VSB DTV transmitter to prescribed states before CCC continues within the M/H Group. The resetting of the twelve 2/3 trellis encoders to place the 12-phase 2/3 trellis encoder in standard starting states is termed “trellis-initialization” in this specification and the claims that follow, no matter when this resetting is performed within an M/H Group. When the most significant bits of 8VSB symbols are subjected to 12-phase pre-coding to compensate for post-comb filtering in receivers, “trellis-initialization” also includes resetting of the 12-phase pre-coder to standard starting states.
The outer convolutional coding does not continue between M/H Groups. So, the outer convolutional coding can be begun again from a prescribed initial state at the beginning of each M/H Group. Neither the outer convolutional coding nor the 2/3 trellis coding used as the inner outer convolutional coding are specified in A/153 as being terminated to prescribed states or as using tail-biting. This reduces the efficiency of turbo decoding procedures and will often increase the number of iterations required to suppress decoding errors. The literature describes turbo decoding being done on a bi-directional basis, reversing the direction of decoding every iteration, which is reported to facilitate selection of the most likely codeword in portions of the signal where certainty is below average. Reverse-direction decoding is easier if zero-flushing or tail-biting procedures terminate the CCC, so the receiver has full knowledge of the final decoding states, rather than having to guess them. Termination of CCC is of less concern when the coding is of extended duration, which is the case in M/H. Employing reverse-direction decoding when turbo decoding the CCC within a Group is more complicated, however, because the receiver has to work backward from a plurality of possible concluding states, rather than from a single concluding state that is prescribed.
The inventor discerned that the conclusion of the inner convolutional coding can also be terminated by modifying certain bytes at the beginning of each M/H Group to reset the 12-phase 2/3 trellis encoder at the 8VSB DTV transmitter so as to have prescribed states thereafter. The inventor observed that there were dummy bytes in each M/H Group as CCC therein concluded. The inventor ascertained that these dummy bytes are so positioned within the M/H Group that there is the possibility that they can be modified to reset the 12-phase 2/3 trellis encoder at the 8VSB DTV transmitter to prescribed states immediately after CCC concludes within the M/H Group.
The inventor discerned that resetting the 12-phase encoder for 2/3 trellis coding to standard states both at the beginning of each M/H Group and at the conclusion of each M/H Group is a practical way to implement tail-biting of the inner convolutional coding. This procedure spares the M/H transmitter apparatus having to store the states of the 12-phase 2/3 trellis coding at the beginning of each M/H Group until the conclusion of that Group. This procedure facilitates burst reception of M/H Groups by M/H receiver apparatus. The receiver when it is powered up at the beginning of each M/H Group will reliably know a priori the states of the 12-phase 2/3 trellis coding and can begin its decoding straightaway. The memory associated with the 12-phase decoder for the 2/3 trellis coding can select just the M/H Group for temporary storage to support turbo decoding.
Trellis-initialization is not suited to terminating the conclusion of the outer convolutional coding transmitted within an M/H Group. This is because of the time order of the 2-bit symbols of the outer convolutional coding being shuffled before the inner convolutional coding. Instead, termination of the outer convolutional coding at its conclusion is done by zero-flushing or tail-biting methods performed by the encoder for the outer convolutional coding.
The inventor discerned that trellis-initialization can be used to partition an M/H Group into first portion and second portions, which first portion can be decoded to supply part of a primary RS Frame and which second portion can be decoded to supply part of a secondary RS Frame. Preferably, the outer convolutional coding in the first portion of the M/H Group is terminated at its conclusion, and the outer convolutional coding in the second portion of the M/H Group is terminated at its conclusion. The M/H data transmitted in the each RS Frame are randomized independently of Mill data in other RS Frames.
The locations where trellis-initialization is to occur can be chosen such that the sizes of the primary and secondary RS Frames are in a simple ratio that facilitates the efficient packing of both those RS Frames with TRS codewords of a common length. Arranging the size of the primary RS Frames to be twice or thrice the size of RS secondary frames can be advantageous for developing programming for M/H transmission, since the lengths of the DTV programs can be standardized to facilitate scheduling.
It is advantageous to use the trellis-initialization preceding the earlier training signal in the M/H Group as one of the two trellis-initializations used for separating the primary and secondary RS Frames. Decoding of the first portion of the M/H Group begins immediately after the earlier training signal and concludes just after the later trellis-initialization between the first and second portions of the M/H Group. Decoding of the first portion of the M/H Group then begins just after the later trellis-initialization. The trellis-initialization at the conclusion of the M/H Group and the trellis-initialization at the beginning of the M/H Group allow the second portion of the M/H Group to be wrapped around. The second portion of the M/H Group then concludes with the trellis-initialization preceding the earlier training signal in the M/H Group.
If the Mill Group encodes M/H data just for a primary RS Frame and not for a secondary RS Frame, the encoding of the M/H data can begin soon after the trellis-initialization at the beginning of the M/H Group and conclude with the trellis-initialization at the conclusion of the M/H Group. Alternatively, the encoding of the M/H data can begin immediately after the earlier training signal and conclude with the trellis-initialization preceding the earlier training signal in the M/H Group. That is, there will be wrap-around of the encoding from the trellis-initialization at the conclusion of the M/H Group to soon after the trellis-initialization at the beginning of the M/H Group. This latter alternative may facilitate more uniform turbo decoding procedures whether or not secondary RS Frames are used.